1. Field of the Invention
The present invention relates to multifunctional composite materials, and methods of making the same. It particularly relates to structural composite materials exhibiting power storage functionality, especially fibre reinforced composite (FRC) materials with integrated energy storage devices, and methods for their production.
2. Description of the Related Art
In many modern structures use is made of fibre/matrix composite materials and in particular fibre/resin composite materials. These are frequently fabricated from one or more cloth layers of a fibre, such as, for example, glass, Kevlar®, Dyneema®, or carbon fibres, formed into a woven fabric or a mat, together with some permeating polymeric resin matrix. However, the permeating matrix may also be provided by the use of pre-impregnated cloth layers “prepegs” or by infusable thermoplastic films interleaved alternately between untreated cloth layers.
Applicant's earlier patent application WO 96/38025 is directed towards a laminated composite material comprising a plurality of cloth layers and a penetrating resin matrix material, wherein at least one of the cloth layers has deposited thereon a patterned layer of electrical conductor, which can be produced by screen printing of conducting inks and is capable of providing electrical connectivity to an embedded electronic device.
In recent years, attempts have been made to develop multifunctional composite materials in order to optimise system performance and, in particular, to develop electric energy storage devices that also perform a structural function. For example, in a micro air vehicle (MAV), a device such as a battery, supercapacitor or fuel cell will add weight and bulk to the MAV and, in fact require the structure to support that device. If, instead, a multifunctional energy storage device can be designed that contributes rather than detracts from the mechanical strength or stiffness of the MAV, considerable size or weight savings may be possible. DARPA funded a program, as described in the article: American Society for Composites (ASC), 16th Technical Conference Proceedings CD-ROM, M. W. Hyer and A. C. Loos, Eds., Virginia Tech, Blacksburg, Va., Sep. 9-12, 2001., which developed a structure battery system for MAV's, which system was based on PLiON™ battery technology. The PLiON cells comprised a laminate of active electrodes and polymer matrix containing liquid electrolyte, which were sealed in laminated polymer/metal chemical barrier layer enclosures in view of the moisture sensitivity of the Li-ion electrolyte. The PLiON materials were then reinforced with compatible structural materials to improve mechanical strength. This is an example where structure function was added onto a known battery functionality.
In 2000, ITN Energy Systems, Inc. developed “PowerFibres”™, which are solid-state thin-film batteries deposited upon individual fibre substrates. The thin component layers are built up by vacuum deposition with a lithium phosphorous oxynitride glass electrolyte known as “LIPON”™ used as the solid electrolyte. Similarly, the US Army Research Laboratory proposed the use of structural polymeric composites based on solid electrolytes. Their battery functionality comprised carbon fibre fabric anodes, fibreglass separator layers, cathode coated metal meshes and a structural solid polymer electrolyte binding the components together so as to transfer load to the other components. WO 2007/125282 proposes the use of a structural supercapacitor in which the electrodes are formed from mats of conducting fibres bound by an electrolytic resin. However, the combining of electrical and structural functions can often lead to poor electrical capacities, in view of the use of non-optimal cell components.
In relation to cell manufacture, it is known to use screen printing techniques for the fabrication of cell elements. U.S. Pat. No. 5,540,742, for example, is directed to a method of fabricating thin, flexible cells involving screen printing various cell elements onto foil layers. EP 0 814 520 discloses printing a thin layer of separator precursor solution onto a cell electrode and drying and curing the layer to form a microporous separator layer.